Obesity (body mass index, BMI >25) afflicts millions of people in the United States (an estimated 97 million adults in the U.S.) and other countries, and is a major risk factor for heart disease, type II diabetes mellitus, stroke, hypertension, and morbidity. In industrialized countries, the problem of obesity is compounded by overeating, a high fat content diet, and a lack of exercise. The last few years have seen the characterization of over 25 neuroendocrine pathways that have been identified to participate in and regulate feeding behavior and energy homeostasis. The melanocortin pathway includes five such genetic factors that have been demonstrated to mediate weight homeostasis, and when modified, result in obesity. The melanocortin pathway includes the melanocortin agonists, derived from the preprohormone proopiomelanocortin (POMC) gene transcript, the five melanocortin receptors identified to date (MC1R-MC5R), and the only 2 naturally occurring antagonists of GPCRs, agouti (ASP) and agouti-related protein (AGRP). The five melanocortin genetic factors identified as being involved in energy homeostasis are POMC, ASP, AGRP, the brain melanocortin-4 receptor (MC4R), and the melanocortin-3 receptor (MC3R). Genetic studies in humans (ca 1500 individuals with severe early-onset obesity BMI > 30) identified 40 naturally occurring heterozygous MC4R mutations, resulting in an unusually high frequency (4%) of heterozygous polymorphisms. Additionally, polymorphisms of obese humans were identified in the POMC gene (MC4R agonists), and polymorphisms of the AGRP (MC4R antagonist) were identified in human patients with Anorexia Nervosa. These data support the hypothesis that the melanocortin-4 receptor and its endogenous agonists (POMC derived) and antagonist (AGRP) are involved in the regulation of feeding behavior and obesity. The overall objectives of this proposal are to 1) characterize these MC4R polymorphisms in vitro to identify which of these mutations results in altered ligand binding or functional activity of either the endogenous agonist (POMC derived peptides) or antagonist (AGRP) 2) identify which polymorphisms modify cell surface localization of the MC4R, and 3) determine if peptides and MC4R small molecule agonists are potential therapeutic avenues for obese humans containing hMC4R protein polymorphisms. Understanding obesity related mechanisms may ultimately result in therapeutic agents to prevent or treat the diseases associated with over eating.